do wide field eps reduce bightness of objects

[Paddy (Patrick)]

I wonder if someone who knows about optics could answer this question for me. Does an object viewed through a wide field ep show less brightness than one viewed through say a plossl? My curiosity is that if a the lens is limited by size, say 1 1/4", does the light get spread over a larger FOV thereby diminishing the light for each object in the FOV. To test this I compared some faint stars through my 9mm Nagler with the same stars with the 9mm plossl that came with the scope and they did seem slightly brighter in the plossl. Is this my imagination, the effect of fewer bits of glass in the plossl, reduced contrast because there are more objects in the FOV of the nagler or because the larger FOV means less light for each object. I love my widefield eps, but wonder if using plossls might be better for very faint objects.

[koputai (Jason)]

Interesting you should bring this up.
I was out a couple of nights ago, on the front porch here in suburban Sydney, and was comparing my Nagler 13T6 (82 AFOV) to my old Unitron 12.5 Ortho (45 AFOV). In the marginal conditions, I could just see the fifth star in the trapezium in the ortho, but not at all in the Nagler.

Cheers,
Jason.

[wasyoungonce (Brendan)]

I thought that higher powered eyepieces (RE:short Focal Length eyepieces) produced less image intensity due to the smaller field stop and area of lenses????

Subsequently, wide field low power EPs should have a brighter image compared with 45degree Plossls, with regard to field stop size and area of lenses??

But there again the image plane is (usually) bigger with wide field EPs so...maybe the intensity of light is spread over this plane..a greater area.

Higher power also produces a larger image (lets say with Jupiter) and the target image intensity is thus spread over a larger area, thus it appears less bright?

Anyone else?

edit;

yep image intensity at focal plane (for large objects) declines as the square of magnification. This shouldn't effect star viewing though as they are point sources.

reference:
http://www.mira.org/ascc/pages/lectures/eyepiece.htm

[Paddy (Patrick)]

Thanks for your replies guys! It's interesting to know that I'm not the only one to have noticed this. Wasyoungonce, I also followed the link you provided and found it an interesting read with a few different perspectives on eyepiece selection. But I still don't think I know the answer to my question. I'm also a little surprised that there haven't been a few more replies with other perspectives.

[ballaratdragons (Ken)]

It may sound contrary to the belief (and mathematics) of what is written above, but I see more nebula in a 2" widefield 30mm than I do in a 1.25mm 30mm plossl! But that could come down to quality of the optics as well.

The theory of light being spread over a larger area being slightly fainter makes sense, but also remember that the eye's pupil has a certain size, and an image larger than this, (exit pupil?) will be dimmer as a percentage isn't even entering the pupil.

Others may help with the exit pupil numbers.

[Astro78]

Still a highly interested newbie (so tread carefully with what i have to say) but from what i've read the proportion of 'illuminated field' is a big play here.

My f5 has a fairly large illuminated field which lights up the 27mm field stop of the 32mm Plossl at virtually 100%. Larger 2" eyepieces offer a larger potential field stop and there will be a point where the 'illuminated field' drops below 100%. Directly effecting brightness.

It is likely that an F6 or higher scope will have a smaller illuminated field , which is prime for high mag and better contrast on the planets (lower obstruction). The higher mag eyepieces have smaller field stops and having a larger illuminated field here is wasted, not all the light will reach the eyepiece.

Anyone who actually knows though please step in....

[jakob]

My theory goes something like that:
The image available at the focal plane of the scope is defined by the mirror or lens (say fl.1830mm F4.5).
The EP is looking at the image available at the focal plane, a short EP looks at a smaller area of the image at the apparent Field of View, a longer EP a larger area.

The brightness of an object at the focal plane is constant but the way the eye is seeing it depens soley on the EP.
For example, with a 26 mm x 68deg. EP the circle of the visible image is 35mm.
A 13mm x 68deg EP will show 17.5mm of the image.
The object will appear twice the size but only 1/4 as bright.

The rest is in the quality and design of the EP.

[skies2clear]

Originally Posted by Paddy

My curiosity is that if a the lens is limited by size, say 1 1/4", does the light get spread over a larger FOV thereby diminishing the light for each object in the FOV. To test this I compared some faint stars through my 9mm Nagler with the same stars with the 9mm plossl that came with the scope and they did seem slightly brighter in the plossl. Is this my imagination, the effect of fewer bits of glass in the plossl, reduced contrast because there are more objects in the FOV of the nagler or because the larger FOV means less light for each object. I love my widefield eps, but wonder if using plossls might be better for very faint objects.

Paddy, what you are seeing is quite common, and the reason for this is better transmission of light through less lenses in the Plossl. The wider field hasn't much to do with it, unless the field isn't fully illuminated because of a restriction somewhere in the light path of the telescope system. This assumes of course, similar quality optics, as in this case you are comparing a TVue with another TVue. This comparison is only valid for the same focal length eyepieces and hence, same magnification. If the optics have better transmission of light, then all objects should appear brighter (both point sources like stars and extended objects, like planets, nebula, etc).

However, there are instances where widefields give a brighter image than narrow fields, but this is because the optical quality of the lenses, ie. type of glass and coatings, is better in the widefield. Again, only comparing the same magnifications, otherwise, other factors, like exit pupil play a part. For example, comparing a Pentax XW (70 deg field) to a Unitron Ortho, of the same focal length, will show the Pentax has brighter images (better glass and coatings. This I have compared by the way, not just theory.

Others please continue....

Cheers
Nick

[Paddy (Patrick)]

Thanks chaps, I feel like I am starting to understand. However, my general experience of astronomy is that this is likely to be a temporary state - which is fantastic.